Seminars

Atmosphere Ocean Science Colloquium

Probing the drivers of the Northern Hemispheric response to Arctic sea ice loss

Speaker:
Paul Kushner, University of Toronto

Location:
Warren Weaver Hall 1302

Date:
Wednesday, December 5, 2018, 3:30 p.m.

Synopsis:

In several modelling studies, consistent coupled atmosphere-ocean general circulation model (AOGCM) responses to induced Arctic sea ice loss have begun to emerge. Robust features to Arctic sea ice loss include tropical and extratropical warming in addition to warming throughout the Arctic troposphere; and an intensification of the wintertime Aleutian Low with an expansion of the Siberian High. If the effects of tropical adjustment (Deser et al.’s so-caled ‘mini global warming’) are accounted for, the isolated circulation response to sea ice loss becomes even more robust.

We suggest that midlatitude ocean warming, that is itself associated with sea ice loss, can account for several of the more robust aspects of the extratropical atmospheric response. This process is probed by extracting the midlatitude sea surface temperature (SST) response to sea ice loss from AOGCM simulations and imposing it as a separate boundary forcing in an atmospheric general circulation model (AGCM), in addition to the loss of sea ice itself. Sea ice loss in isolation drives warming that is confined to the Arctic lower troposphere and only a weak atmospheric circulation response. When the extratropical SST response caused by sea ice loss is also included in the forcing, the warming extends into the Arctic mid-troposphere during winter and the circulation response strengthens in a manner more consistent with AOGCM experiments. This result is robust to different estimation methods for the SST response to sea ice loss.

These results help explain why AGCM experiments that are driven by sea ice loss, even if they include local SST warming in the sea ice loss regions, underestimate Arctic free tropospheric warming seen in coupled atmosphere-ocean GCM simulations. Important questions remain about the dynamical drivers of the extratropical SST warming used as a forcing here, which point to the need for careful experimental design in future model studies.